Initially, the United States Post Office Department (dissolved and replaced with the USPS in 1971) created a two-digit code in 1943, assigning numbers to the country's largest cities. In 1963, the Post Office replaced that with the 5-digit system which became mandatory for second and third class bulk mailers in 1967.

Although it wasn't mandatory for everyone, the USPOD hoped all Americans would adapt to the new system quickly. To assist with that, the department created a series of promotional films and a mascot to accompany the campaign named "Mr. Zip."

This brief video (with a Mr. Zip cameo) breaks down the code system, showing how each digit represents either a region (the first number), a city (the next two numbers) and the post office that the parcel is being sent to (the last two numbers):

But no USPOD PSA showed nearly as much enthusiasm for the new ZIP code system as much as its 15 minute, "Swinging Six" film created in 1967:

The video shifts back and forth between a narrator and a folk-singing sextet, each preaching the virtues of the "five trailblazing numbers," that "launch every piece of mail with space age speed and precision."

The folk singers share stories that help us see how ZIP codes affect our personal lives. One story is about a New York City man in long-distance relationships with women from "Frisco Bay." In the pre-ZIP code world, letters fail to arrive in time, leaving the woman no choice but to start dating local men. He learns his lesson however, using a ZIP code for his letters to a new love; these letters arrive faster and prevent "Frisco Bay" competition from swooping in.

Another musical number later in the film reminds us that we have terrible handwriting, confusing postal workers and increasing the chances of letters not reaching the intended destination. The obvious solution? The ZIP code, of course, as well as the equally new, two letter abbreviation for each state.

50 years since the introduction of the ZIP code, its purpose stretches well beyond efficient mail delivery, becoming an indicator of how wealthy we are, how much crime is in our neighborhoods, or even whether or not we'll qualify for food stamps after a natural disaster hits us. Those five numbers have ended up meaning a lot more to Americans than Mr. Zip or the "Swinging Six" ever imagined.